Set PTE_A and PTE_D for user mappings in pmap_enter().

This assumes that an access according to the prot in 'flags' triggered
a fault and is going to be retried after the fault returns, so the two
flags are set preemptively to avoid refaulting on the retry.

While here, only bother setting PTE_D for kernel mappings in pmap_enter
for writable mappings.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D17782